1. Field of the Invention
The present invention relates to an image recording apparatus using a plurality of diffraction grating type light modulator elements for recording an image on a recording medium.
2. Description of the Background Art
Developed has been a diffraction grating type light modulator element which is capable of changing the depth of grating by alternately forming fixed ribbons and moving ribbons on a substrate with a semiconductor device manufacturing technique and sagging the moving ribbons relatively to the fixed ribbons. It is proposed that such a diffraction grating as above, in which the intensities of a normally reflected light beam and diffracted light beams are changed by changing the depth of grating, should be used for an image recording apparatus in techniques such as CTP (Computer to Plate) as a switching element of light.
For example, a plurality of diffraction grating type light modulator elements provided in the image recording apparatus are irradiated with light, and then reflected light beams (zeroth order diffracted light beams) from light modulator elements in a state where the fixed ribbons and the moving ribbons are positioned at the same height from a base surface are guided to the recording medium and non-zeroth order diffracted light beams (mainly first order diffracted light beams) from light modulator elements in a state where the moving ribbons are sagged are blocked, to achieve an image recording on the recording medium.
In such a diffraction grating type light modulator element, however, since the driving voltage supplied for the moving ribbons and the amount of sag of the moving ribbons are not in proportion to each other, even if a curve indicating a change in driving voltage at the time when the light modulator element is changed from an ON state (a state where a signal beam is guided from the light modulator element to the recording medium) to an OFF state (a state where no light is guided from the light modulator element to the recording medium) is made equivalent (symmetrical) to a curve indicating a change in driving voltage at the time when the light modulator element is changed from the OFF state to the ON state, changes in intensity of light outputted from the light modulator element in both the cases do not become equivalent (symmetrical) to each other.
Specifically, when the light modulator element is changed from a state where the change in sag of the moving ribbons is large with respect to the change in driving voltage to a state where the change in sag is small, it is hard for the moving ribbons to follow the driving voltage since a large initial acceleration is given to the moving ribbons and this results in excessively quick moving of the moving ribbons and oscillation thereof. As a result, even if the light modulator elements are changed periodically between the ON state and the OFF state, it is hard to write appropriate dots on the recording medium which travels at constant speed relatively to the light modulator elements.